Optical resonators have been used as sensor elements by exposing the respective resonator to an observable and by detecting shifts of resonance wavelengths caused by the observable.
In this context, optical micro ring resonators have a great importance due to their high quality as optical sensors. A sensor of this type is very sensitive as a surface of the micro ring is scanned by an evanescent field of a light wave propagating within the micro ring. Currently, micro ring resonators are used to perform measurements with a selectively working absorber surface, which plays an important role for an adequate specificity of the sensor. Molecules adsorbed or collected at the surface cause an optical ring circumference to vary. Thus, an effective refractive index of the micro ring resonator changes such that wavelengths of resonant modes are shifted. The goal is to reliably measure changes in the refractive index of the surrounding medium even if their relative size is far less than 10−5. Similarly, micro ring resonators can also be used for measuring temperature effects (this is possible if resonance shifts are caused by a temperature dependence of the refractive index), mechanical influences (e.g. via stress birefringence) or humidity (as water will condensate on a surface of the micro ring and lead to a change in the effective refractive index).
In order to achieve sufficiently high resolutions, known sensors of this type will need a tuneable diode laser. This is due to the fact that a wavelength resolution of the sensor arrangement is directly related to the sensor sensitivity. Problems may be caused by the fact that tuneable diode lasers are very expensive.